admin: Use really persistent d-bus name

Commit 8e9439ef introduced DBusName=org.gtk.vfs.mountpoint_admin
in admin.mount.in, but forgot to set the necessary mount options.
So, each client spawns new daemon currently, which is not necessary.
Let's set the missing -DMOUNTABLE_DBUS_NAME options.

https://bugzilla.gnome.org/show_bug.cgi?id=793445